Generally a sewing machine is designed to produce stitches with an upper thread (needle thread) and a lower thread which are locked with each other to form generally so called lock stitches. Especially in case of the straight stitches, if the upper and lower threads are normally locked as shown in FIG. 10, the normal lock stitches are formed as desired. On the other hand, if the upper and lower threads happen to be irregularly locked as shown in FIG. 11, the irregular lock stitches, which are generally called hitch stitches, are formed. If the hitch stitches are mixed in a series of the normal lock stitches, the stitches will lack uniformity and will have a lower quality of stitches as shown in FIG. 14.
Further the normal lock stitches are the products of the upper and lower threads locked with each other in accordance with the twist of the threads. On the other hand, the hitch stitches are the products of the upper and lower threads locked with each other in a manner as to untwist the thread, especially the upper thread.
The formation of normal stitches or hitch stitches depends on a position of a lower thread with respect to a needle which drops to one point at all times. As shown in FIG. 12, if the lower thread 40 is positioned on the left side of the needle from a view of a machine operator, the loop taker is rotated counter-clockwise as shown by an arrow, and catches a needle thread loop formed on the right side of the lower thread and locks the needle (upper) thread to the lower thread through a stitch formation phases, FIG. 12(a)-(g). As the result, the normal stitch is formed. On the other hand, if the lower thread is positioned on the right side of the needle as shown in FIG. 13, the loop taker is rotated to catch a needle thread loop formed on the left side of the lower thread and locks the needle thread to the lower thread through a stitch formation phases, FIG. 13(a)-(g). As the result, the hitch stitch is formed.
It is therefore required to provide the condition in FIG. 12 to secure the formation of the normal stitches at all times. This requirement may be satisfied by supplying the lower thread in a manner that the lower thread is drawn out from the bobbin carrier at a cut-out as shown in FIGS. 1 and 3, which is located leftward of the left end of a laterally elongated needle dropping hole of a needle plate.
As particularly shown in FIG. 15(a), the lower thread is supplied to the left end L of the laterally elongated needle dropping hole from a position opposite to the needle hole and leftward of the left L of the needle hole, because the straight stitches are generally formed with a needle position set adjacent to the left end L of the laterally elongated needle hole 18a within which the needle is laterally swingable from minimum to maximum for zigzag stitches. The needle position adjacent to the left and L of the needle hole is so set as to reduce the up and down movements of a fabric which may otherwise be caused as the needle penetrates into and out of the fabric to be sewn.
However with references to FIG. 15(a), in case of zigzag stitching, the needle is swingable between the opposite needle positions L and R over a distance D. It is therefore observed that the required amount of lower thread is different depending upon the needle positions L and R. Much more amount of lower thread is required when the needle come to the right end needle position R. The thread tension due to drawing out an additional amount of lower thread will pull the needle thread down onto the underside of the fabric to be sewn in contrast to the right-side lock of threads as shown in FIG. 16. Namely the tread locking positions are unblanced.
In order to form the zigzag stitches of balanced thread locking positions as shown in FIG. 17, it is required to provide a condition for supplying the same amount of lower thread to both needle positions L and R from the lower source, that is, a condition as shown in FIG. 15(b), providing the thread paths of a same distance from a thread supply to the needle positions L and R, respectively.
Japanese utility model laid-open application 56-23074 discloses a sewing machine having a lower thread normally drawn out from a thread source through an opening located at a position corresponding to a center of a laterally elongated needle dropping hole of a needle plate, wherein a thread guide member is operatively connected to a pattern selecting device including a plurality of pattern selecting keys, and is operated in response to selection of a straight stitch pattern to guide the lower thread toward a left side with respect to the left end of the laterally elongated needle dropping hole from a view of a machine operator.
The prior art is actually successful in preventing the formation of hitch stitches in connection with the straight stitches. However as far as the zigzag stitches are concerned, which require the needle thread to extend laterally of the fabric feeding direction, the fabric is easily shrunk in the lateral direction due to the zigzag stitches if the lower thread tension is kept the same as in the case of the straight stitches. It is therefore required to separately adjust the upper thread tension to prevent such shrinkage of fabric. It is, however, insufficient to completely prevent such shrinking phenomena of fabric only by separately adjusting the upper thread tension.